This application relates generally to spark plugs for internal combustion engines and, more particularly, to a ground shield for a spark plug having an annular threaded portion for engaging the engine with a spark plug seat that is located between the spark gap and the threaded portion.
Traditional spark plug construction includes an annular metal casing having threads near one end and a ceramic insulator extending from the threaded end through the metal casing and beyond the opposite end. A central electrode is exposed near the threaded end and is electrically connected through the insulator interior to a terminal which extends from the opposite insulator end to which a spark plug ignition wire attaches. A “J” shaped ground electrode extends from one edge of the threaded end of the metal casing into axial alignment with the central electrode to define a spark gap therebetween. The force applied to seal the spark plug in the head is the result of torque transmitted by the threaded metal casing; hence, the threaded portion of the metal casing must be sturdy and of substantial size. A portion of the metal casing is formed to be engaged by a socket tool to provide torque to the threaded portion. The threaded portion is located away from the portion which is engaged by the socket tool.
To facilitate the controlled and efficient exhaust of gases from a combustion chamber, the valves are sometimes increased in size. This may necessitate a decrease in the size of the spark plug, a reduction in the size and sturdiness of the threaded metal casing end, and, in particular, a decrease in the inside diameter of the metal bore of the spark plug and in the combustion chamber wall area available to threadedly receive the spark plug.
The decrease in the inside diameter of the metal bore of the spark plug reduces the ability of the spark plug to resist carbon build up and similar deposits reducing ignition efficiency. Various designs for spark plugs that reduce the deleterious effect of reducing the spark plug size by having an insulator with a cylindrical body that surrounds a central electrode are taught in U.S. Pat. Nos. 5,091,672, 5,697,334, 5,918,571, and 6,104,130, the contents for each incorporated herein by reference. In these designs, the cylindrical body is provided with a first diameter section separated from a second diameter section by a shoulder that provides a surface for sealing to the engine cylinder head. A shield that surrounds the second diameter has a base portion that is positioned a fixed distance from the tip to the center electrode by the engagement of a flange on the shield with the shoulder on the cylindrical body. The shield is formed with a ground electrode that integrally extends from the base portion. A shell portion surrounds the first diameter section of the cylindrical body and contains a threaded section positioned higher than the cylinder head seating surface along the cylindrical body. A radial tab extends from an end of the shell and aligns with the flange within the head to establish uniform positioning of the base portion. A separate end or retainer nut extends from the opposing end of the shell to locate and position the spark plug within the combustion chamber.
Particularly suited for high-compression, high-performing engines, these various high-thread spark plug designs can provide more power by allowing for more space to optimize engine design, a superior cylinder head-seating position, a more compressive seal, improved heat transfer, and a more stable spark plug operating temperature for a more focused ignition, as well as a longer service life and increased corrosion protection. Nevertheless, to maintain the sparking gap between the center electrode and the ground electrode, the ground shield must be manufactured from an expensive, proprietary nickel alloy material.
Accordingly, the inventor herein has recognized that it is desirable to provide a cost effective ground shield for use in a high-thread spark plug structure.